1. Field of the Invention
The invention is generally related to the area of optical devices. In particular, the present invention is related to compact passive modules for erbium and other doped fiber amplifiers.
2. The Background of Related Art
EDFA (erbium-doped fiber amplifier) is an optical repeater device that is used to boost the intensity of optical signals being carried through a fiber optic communications system. An optical fiber is doped with the rare earth element erbium so that the glass fiber can absorb light at one frequency and emit light at another frequency. An external semiconductor (or pump) laser couples light into the fiber at infrared wavelengths of either 980 or 1480 nanometers. This action excites the erbium atoms. Additional optical signals at wavelengths between 1530 and 1620 nanometers enter the fiber and stimulate the excited erbium atoms to emit photons at the same wavelength as the incoming signal. This action amplifies a weak optical signal to a higher power, effecting a boost in the signal strength.
FIG. 1 shows an exemplary implementation 100 of using an EDFA. A signal (e.g., around 1550 nanometers) 102 is coupled to a tap coupler 104 that takes a sample of the signal 102 to be tested via a photo-diode 106. The signal 102 goes through an isolator 108 that allows a light beam to go one way. A Wavelength Division Multiplexing (WDM) filter 110 is provided to couple an external laser (e.g., at 980 nanometers) into the signal 102 that then goes through an EDFA 114 to be amplified. A second WDM 110 is provided at the same time to couple an external laser (e.g., at 1480 nanometers) into the signal for amplification in the EDFA 114. The amplified signal goes through another isolator 118 before it reaches a gain flattening filter (GFF) 120 which the signal is adjusted. Another tap coupler 122 is provided to take a sample of the signal for test via a photo-diode 124. The amplified signal is coupled out by a collimator 126.
In summary, the implementation 100 of FIG. 1 includes many components. In addition to the erbium doped fiber amplifier, there are a 980 nm pump laser and a 1480 nm reverse pump laser, three tap couplers with photo-diodes for input and output power monitoring, a 980 nm WDM filter and a 1480 nm WDM filter for multiplexing the pump and amplifying laser signals, a GFF for gain curve correction, and two isolators. It can be noticed, however, that there are 12 inter-component fiber splicing joints that are needed to connect all the components, which requires extensive laboring to put the fiber splicing joints together with the components. Further, it may be observed that the implementation can be expensive in cost and fragile in operation.
Accordingly, there is a great need for different techniques to realize optical amplifiers. Such devices so designed are amenable to small footprint, broad operating wavelength range, enhanced impact performance, lower cost packaging, and easier manufacturing process.